This invention relates to a signal detector used for an information recording/reproduction device and more particularly to a signal detector in which an element for detecting a signal is easily assembled.
A type of conventional signal detector has a semiconductor laser for emitting laser light, a beam splitter through which laser light from the semiconductor laser travels to an optical disk and which splits signal light reflected and diffracted by the optical disk, an objective lens for condensing and projecting the laser light transmitted through the beam splitter to an information recording surface of the optical disk, a biprism for separating the signal light split by the beam splitter into two beams, and a photodetector device which has a plurality of photodiodes for receiving the two beams of signal light separated by the biprism and which detects an error in focusing on the optical disk.
The operation of the conventional signal detector thus constructed will be described below.
Laser light emitted from the semiconductor laser is projected to the information recording surface of the optical disk by the condensing effect of the optical system. The projected laser light is reflected and diffracted by the information recording surface of the optical disk to form signal light. This signal light is separated into two beams of light by the biprism, and these two beams of light are received by the photodiodes of the photodetector device bisectionally arranged.
Defocus detection is effected based on the Foucault method using signals output from the photodiodes receiving the light to obtain a focus error signal.
It is thereby possible to constantly maintain the distance between the objective lens and the information recording surface of the optical disk.
The signal detector in accordance with the prior art, however, entails a problem in that the photodiodes of the photodetector device and the biprism must be attached with a high accuracy while moving and adjusting the attached positions so that the beams of signal light condensed and projected at the time of focusing are correctly condensed to the photodiode division lines. Also, in a case where attachment adjustment is not effected, it is necessary to separately provide an error correction circuit.